Percussion, rotation, and percussion with simultaneous rotation variants of equipment for the treatment of rock and concrete are available. It is well known that the impact mechanism in such equipment is driven hydraulically. A hammer piston, mounted such that it can move in a cylinder bore in a machine housing, is then exposed to alternating pressure such that a reciprocating motion of the hammer piston in the cylinder bore is achieved. The alternating pressure is most often obtained through a separate switch-over valve, normally of gated type and controlled by the position of the hammer piston in the cylinder bore, couples alternately to at least one of two drive chambers, formed between the hammer piston and the cylinder bore, to a line in the machine housing with driving fluid, normally hydraulic fluid, under pressure, and subsequently to a drainage line for driving fluid in the machine housing. A periodically alternating pressure arises in this manner, with a periodicity that corresponds to the impact frequency of the impact mechanism.
The manufacture of gate valveless impact mechanisms, also known as valveless mechanisms, has also been known for more than 30 years. Instead of having a separate switch-over valve, the hammer piston in valveless impact mechanisms is caused to perform also the work of the switch-over valve through it opening and closing for the supply and drainage of driving fluid under pressure during its motion in the cylinder bore in a manner that provides an alternating pressure as described above, in at least one of two drive chambers separated by a drive part of the hammer piston. One condition required for this to work is that channels, arranged in the machine housing for the pressurisation and drainage of a chamber, open out into the cylinder bore such that the openings are separated in such a manner that short-circuiting connection does not arise directly between supply channel and drainage channel at any position of the reciprocating motion of the piston. The connection between the supply channel and the drainage channel is normally present solely through the gap seal that is formed between the drive part and the cylinder bore. If this were not the case, large losses would arise, since driving fluid would be allowed to pass directly from high-pressure pump to tank without any useful work being carried out.
In order for it to be possible for the piston to continue its motion from the moment at which a channel for the drainage of a drive chamber is closed until a channel for pressurisation of the same drive chamber is opened, it is necessary that the pressure in the drive chamber is changed slowly as a consequence of a change in volume. This can take place through the volume of at least one drive chamber being made large relative to what is normal for traditional impact mechanisms of gate valve type. It is necessary that the volume be large since the hydraulic fluid that is normally used has a low compressibility. We then define the compressibility, κ, as the ratio between the relative change in volume and the change in pressure, as follows: κ=(dV/V)/dP. It is, however, more usual to use the modulus of compressibility, β, which is the inverse of the compressibility as we have defined it above, as a measure of compressibility. Thus β=dP/(dV/V). The units of measurement of the modulus of compressibility are Pascal.
The volume is to be sufficiently large that the pressure in the chamber during the change in volume that the chamber experiences during the motion of the hammer piston towards the opening of the channel for pressurisation of the chamber will not be sufficiently large to reverse the motion of the piston before the channel has opened.
A valveless hydraulic impact mechanism with two drive chambers is known through U.S. Pat. No. 4,282,937, where the pressure alternates in both of these chambers. Both drive chambers have large effective volumes through them being in continuous connection with volumes lying close to the cylinder bore.
A valveless hydraulic impact mechanism according to another principle is known through SU 1068591 A, namely with alternating pressure in the upper drive chamber and constant pressure in the lower, which is the drive chamber that lies most closely to the connection for the tool. In this case, the upper drive chamber, which is the one in which the pressure alternates, has a considerably larger volume than the lower drive chamber, in which the pressure is constant.
One problem with large drive chambers in which the pressure continuously alternates between system pressure and return pressure, i.e. approximately atmospheric pressure, is that the machine housing itself tends to suffer from the formation of cracks as a consequence of material fatigue. In order to avoid this, designs that have thick and complex castings with intermediate walls have until now been required, with a high cost and weight that follow from this.